This project investigates diagnosis and treatment of severe combined immunodeficiency. Affected infants have severe infections that are fatal unless the immune system can be restored. Bone marrow transplant (BMT) is life-saving if the disease is detected in time. SCID is most often caused by defects in the X-linked IL2RG gene which encodes the common gamma chain of receptors for cytokines. When this gene is defective, lymphocytes do not develop normally. To know how mutations in the IL2RG gene cause X-linked SCID (XSCID), we collect samples of blood or tissue, perform DNA analysis, assess expression of common gamma chain protein, and analyze its function. Mutations occur in all exons of the gene. Unusual mutations give clues to how the gene normally interacts with other proteins on the surface of the cell and in the cytoplasm of the cell. Certain mutations permit residual function and have distinct clinical characteristics. We look for other gene defects in patients with SCID but with no mutation in the IL2RG. We perform carrier testing, genetic counseling prenatal diagnosis, making affected infants eligible for improved early treatments. Despite improved survival with BMT, many XSCID patients are not completely cured, raising the question whether retroviral gene transfer ex vivo to autologous blood-forming cells could improve outcome. We have a complete XSCID gene therapy program, including vector development, animal models, retroviral transduction optimization, clinical evaluation of patients who have failed standard BMT treatment, and a clinical gene therapy protocol under which 2 patients have been treated. In a French gene therapy trial for XSCID, 8 patients had immune reconstitution, but 2 developed leukemia due to gene therapy vector insertion near an oncogene. Our trial is to treat only patients who have failed standard bone marrow transplant treatment. SCID is only one of many diseases that could be approached by gene therapy to blood-forming stem cells. Our studies with gene transfer for treating SCID are a pilot application chosen because of the special biology of the IL2RG gene. This gene confers a selective advantage on corrected cells as compared to cells without a functioning gene. Therefore if gene therapy is successful in SCID, the methods will be applicable to further human diseases such as additional SCID due to non-X-linked genes and HIV/AIDS. The true incidence of SCID is unknown because affected infants may die of infections without being diagnosed. Early identification by population-based newborn screening would enable more infants to be saved and would determine the true disease incidence. We have developed a newborn screening test for SCID of any genotype by quantitation of TRECs (T cell receptor excision circels). The test has been validated on blood spots from the Maryland State newborn screening program.